This invention relates generally to diffusion braze repair of superalloy articles and more particularly to cobalt-base braze alloy compositions containing at least one of the following elements: rhenium, palladium, platinum, ruthenium, iridium; and to long term diffusion heat treatment of repaired superalloy articles.
High temperature operating environments such as those present in gas turbine engines, power generation turbines, refinery equipment, and heat exchangers demand parts composed of a variety of cobalt-, iron-, and nickel-base metals known as superalloys. These superalloys are capable of withstanding extremely high temperatures for extended periods of time, but the extremely stressful temperature conditions to which superalloy articles are subjected eventually take their toll upon the metal in a number of ways.
The main types of damage to a superalloy article are cracks from thermal fatigue, wide gap cracks, foreign object impact damage, and dimensional reduction from mechanical wear. Because the cost of these superalloy components is quite high, there is considerable incentive to repair these types of defects rather than to scrap the part and replace it with a new one. The high cost of these components, as well as the fact that superalloy components, once damaged, tend to fail repeatedly in the same region, also makes it critical that any repairs made have mechanical, environmental, and processing properties equivalent to or better than the original superalloy base metal.
Traditional methods for repairing damaged superalloy articles involve choosing or creating an alloyed combination of elements that will melt at a temperature below the melting temperature of the superalloy substrate. These compositions are known in the industry as braze alloys, and the most useful prior art braze alloys are characterized as either nickel-base or cobalt-base alloys. Historically, the most popular braze alloys contain a melting point depressant such as silicon or boron; a complex of some of the same alloying elements used in the superalloy article to be repaired such as chromium, aluminum, titanium, tungsten, etc.; and either nickel or cobalt as the base. In fact, one braze alloy, sometimes known as B-28, is simply the combination of an alloy frequently used to manufacture cast turbine airfoils, named Rene""80, with about 2% boron.
Advances in the braze alloy composition art have introduced multi-constituent alloy compositions that are mixtures of at least one braze alloy and at least one base metal alloy, the base metal alloy differing from the braze alloy in that it melts at a higher temperature than the braze alloy and contains no melting point depressants that can weaken the repair site. These multi-constituent compositions result in stronger repairs because the low-melting brazing alloy liquefies first, wetting the base metal constituent and joining the entire mixture to the superalloy article.
Once a braze alloy or alloy mixture has been chosen, the damaged superalloy article is cleaned to remove any environmental coating that may be over the base metal and any oxides that may have developed inside the damaged regions. The braze alloy composition is then applied to the region to be repaired, and the article subjected to a high temperature brazing cycle to melt and join the braze alloy to the superalloy article. Upon the completion of this cycle, typical braze alloys will have formed undesirable large blocky or script-like brittle phases composed of chromium, titanium, and the family of refractory elements (e.g., tungsten, tantalum) combined with the melting point depressants. These brittle phases weaken the repair composite and cannot be removed from conventional braze alloys.
However, certain braze alloy compositions, known as diffusion braze alloys, are capable of withstanding higher temperatures than conventional braze alloys. Diffusion braze alloys form the same bad phases during brazing as conventional alloys, but diffusion braze alloys can be subjected to a second, long-term high temperature heat cycle known as a diffusion cycle. This diffusion cycle allows the brittle borides, carbides, and silicides to break down into fine, discrete blocky phases. The diffusion cycle also diffuses the elemental melting point depressants into the braze alloy matrix. These actions result in a stronger repair that is less susceptible to incipient melting when the part is returned to service.
Unfortunately, the diffusion braze alloys of the prior art have failed to attain the crucial part-like mechanical and environmental properties demanded by the increased stresses to which today""s superalloy articles are subjected. The main reason for this failure is that prior high temperature braze alloys and alloy powder mixtures tend to use only those elements present in the superalloy article being repaired.
This lack of flexibility in the compositions of the prior art has caused a stagnation in the development of truly new braze alloy compositions which employ elements and elemental combinations without regard to the composition of the superalloy substrate. As well, previous multi-constituent alloy compositions were so precisely matched to the particular superalloy to be repaired that it was considered unthinkable to select base metal powders for the mixture based solely on their mechanical and environmental properties.
For these reasons, prior art compositions cannot provide a flexible diffusion braze alloy system capable of accommodating various new elements and base metal powders to increase the strength, flow characteristics, and oxidation resistance of the braze alloy system. Prior art heat treatment cycles are similarly incapable of effectively breaking down brittle phases and allowing the elemental melting point depressants to diffuse both into the superalloy substrate and the base metal matrix. As well, prior art diffusion braze alloy compositions frequently rely upon intentional carbon additions for strength, and these prior art compositions do not effectively impart improved environmental resistance to the superalloy substrate and/or any environmental coating which may be applied to the substrate.
A need therefore exists for a new diffusion braze alloy system that desirably employs the elements rhenium, platinum, palladium, ruthenium, iridium, and/or aluminum in order to improve significantly over the hot corrosion and oxidation resistance properties provided by prior art braze alloys. Additionally, such an improved braze alloy composition preferably uses boron and silicon concurrently as melting point depressants in order to reduce the undesirable mechanical and environmental properties associated with the use of either boron or silicon alone. The present invention addresses these needs.
Briefly describing one aspect of the present invention, there is provided an improved cobalt-base braze alloy composition and method for diffusion braze repair of superalloy articles that achieves mechanical, processing, and environmental properties equivalent to and, in many cases, better than those properties possessed by the superalloy articles. The present cobalt-base braze alloy composition comprises nickel; at least one element selected from the following group: rhenium, palladium, platinum, ruthenium, iridium; boron; silicon; and cobalt. This composition may also include one or more of the rare earth elements such as yttrium, cerium, lanthanum, and other lanthanide series elements; aluminum; chromium; titanium; tungsten; molybdenum; niobium; hafnium; tantalum; iron; manganese; and/or zirconium, which elements appear in many advanced superalloy base metal compositions. This cobalt-base braze alloy composition may be combined with one or more powdered base metal superalloy compositions to form an improved diffusion braze alloy mixture having enhanced mechanical, environmental, and processing properties compared to prior art braze alloy mixtures. The present invention also provides new cobalt-base base metal alloy compositions for use in such improved diffusion braze alloy mixtures, which base metal alloy compositions do not include melting point depressants but which are otherwise similar to those of the braze alloy compositions.
In the case of non-eutectic alloys according to the present invention, the instant invention employs melting point depressants such as boron, silicon, and aluminum to reduce the melting point of the braze alloy. Although the present braze alloy compositions contain relatively low amounts of melting point depressants, these depressants nonetheless adversely affect the mechanical and/or environmental properties of a repaired article unless they are subjected to a long-term diffusion heat treatment cycle.
The present invention therefore also describes an improved diffusion heat treatment method to break down the undesirable phases formed by the melting point depressant(s) and diffuse the depressant(s) into the base metal alloy matrix. Use of this long-term diffusion heat treatment method minimizes the negative properties associated with the use of conventional melting point depressants.
In the brazing method of the present invention, a damaged region of a superalloy article is repaired by first cleaning the article by any conventional means; preparing a braze alloy composition mixture according to the present invention, wherein the mechanical and environmental properties of that mixture are chosen to equal and preferably improve upon those properties of the superalloy article to be repaired; depositing this mixture on the region to be repaired; and placing the superalloy article in a furnace under an inert gas atmosphere or under a vacuum. Once in such a furnace, the pressure in the furnace chamber should be reduced to approximately 1xc3x9710xe2x88x923 torr or a lower pressure and the brazing cycle initiated by heating the repaired region to a temperature of about 800xc2x0 F. The 800xc2x0 F. temperature is maintained for approximately 15 minutes, whereafter the temperature is increased to about 1800xc2x0 F. and that temperature maintained for approximately 15 minutes. Next, the temperature is again raised to a temperature less than the incipient melting temperature of the article being repaired, which incipient melting temperature typically exceeds 2350xc2x0 F., and that less than incipient melting temperature maintained for between 15 and 45 minutes. Finally, the furnace is vacuum cooled from the less than incipient melting temperature to about 1800xc2x0 F. This step completes the conventional brazing cycle which causes the formation of undesirable brittle phases. The next steps in the present method constitute the diffusion heat treatment cycle that will break down these brittle phases.
Upon completion of the high temperature brazing cycle, the superalloy article is subjected to a pressure higher than the pressure used in the brazing cycle and reheated to a temperature of between 1 and 400xc2x0 F. below the chosen brazing temperature for the article. This temperature is maintained for at least 20 hours, whereafter the temperature is lowered to about 250xc2x0 F. At this point, the superalloy article is fully repaired and ready for machining.
The superalloy article is then usually coated with a metal or ceramic, diffusion or overlay coating according to any known application method. This coating protects the superalloy base metal from oxidation and hot corrosion attack, and, if the superalloy article is given a multi-layer coating of which at least one layer is a cobalt-base braze alloy according to the present invention, the coating remains resistant to environmental attack much longer than a traditional coating.
These and other objects, advantages, and features are accomplished according to the compositions and methods of the following description of the preferred embodiment of the present invention.